Cell death by apoptosis is triggered when cytochrome c is released from the mitochondria to activate the caspase proteases. Our results indicate that mitotic cells and postmitotic neurons regulate cytochrome c-mediated caspase activation in fundamentally different ways. Whereas cytochrome c is sufficient to induce rapid caspase activation and apoptosis in many mitotic cells including naive PC 12 cells, the neuronally-differentiated PC12 cells and primary sympathetic neurons are remarkably resistant to cytochrome c due to strict regulation by the inhibitor of apoptosis proteins (lAPs). Our preliminary data indicate that this differential regulation of apoptosis is not because of any differences in lAPs themselves in mitotic cells and postmitotic neurons, but because of differences in the rate of caspase activation controlled by Apaf-1. Our hypothesis is that a decrease in Apaf-1 function with neuronal differentiation is critical for engaging lAPs to strictly regulate apoptosis in neurons. The aims of this proposal are to test this hypothesis in the model of differentiating PC12 cells and primary neurons with a specific focus on examining how transcriptional and posttranslational changes in Apaf-1 with neuronal differentiation control the selective, strict regulation of apoptosis by lAPs in neurons.